A Kinetic Model for Hydroxyapatite Precipitation in Mineralizing Solutions

Numerous studies have shown that there is an amorphous calcium phosphate (ACP) phase preceding the precipitation of crystalline hydroxyapatite (HA) in calcium phosphate solutions. It has also been shown that the addition of magnesium to the solutions has a stabilizing effect by inhibiting the transformation of ACP to HA. The stabilizing effect of Mg²⁺ is attributed to the stronger bonds between water molecules and the magnesium ions adsorbed on the surface of the ACP particles, making it harder for them to dehydrate. However, the kinetics of the reactions between calcium and phosphate ions to form ACP and then HA crystals, and the effects of varying concentrations of Mg on the kinetics have not been studied theoretically in detail. In this study, we develop and validate a kinetic model for analyzing such reactions. The pertinent rate constants are derived by calibrating the model against temporal changes in Ca²⁺ concentration reported by others. The predicted onset and growth of HA crystallization for solutions with different Mg concentrations are consistent with those measured. As it is capable of predicting the production of ACP and the subsequent transformation to HA under different assumed conditions, the kinetic model developed can help further our understanding of the mechanism of mineralization of calcium phosphate solutions.